Companies can choose to build their own staff pools where you qualify as a talent. Joining a pool means that, along with a number of other talents, you are selected by a company to which they turn if they need staff. In these pools, the company publishes gigs that are only visible to the talents in the pool.
A pool is often created when there are certain requirements, so read the pool's description carefully and make sure you meet any qualification requirements before applying to join.
Each time a pool matches your criteria, you receive a note. Make sure you have a full profile to succeed in joining a pool.